The metaphor levels are at code-red on Grand Central, an overwrought, over-thought tale of leaking passions inside a nuclear power plant. Here come the hard-hat romantics, tramping up the gangway to circle the core. Radiation sickness, they tell themselves, is a little like being kissed in a bar by a beautiful woman. A little giddiness, a fear of what comes next. Ever vigilant, they wear regulation latex to stave off the worst excesses.

Tahar Rahim gives a rumpled, compelling performance as Gary, an unschooled McJobber who takes a gig as a decontamination sub-contractor, carted like cattle from one reactor to the next. The employees board at makeshift campsites behind the smokestacks and it is here that Gary slips into an ill-starred affair with Karole (Léa Seydoux), the libidinous fiancee of an alpha-male co-worker (Denis Menochet). At clocking-off time, the yarn has them scurrying off for moonlit trysts by the river, amid the bull-rushes, where the crickets chirp and the cellos croak.

Grand Central, which screens in Un Certain Regard at this year's Cannes film festival, makes a great show of its blue-collar pedigree and wanton, boozy antics. The film has tobacco on its breath and sweat-rings at its armpits, although it's not as brawny as it would have us believe. The drama, we realise, is running a fever. That undigested romantic subplot has fogged its senses and weakened its knees.

Rebecca Zlotowski's picture works best when it's at its most stark and unadorned, when it focuses on the dirty nuts and bolts near the nuclear core. Along the way, Grand Central lifts the lid on a high-stakes Catch-22 system in which employees have to keep their radiation levels down for fear of being laid off - a devastating sleight of hand that effectively makes the sub-contractors responsible for their own safety. Right near the end we see a contaminated woman being dragged into the green-zone: her body doused, her head forcibly shaved. The prognosis looks bleak. But wait - is this woman sobbing because she is scared of losing her life or scared of losing her job? Zlotowski, smartly, leaves us to decide that for ourselves.

As Cannes swings into full-throttle funhouse mode, there is much to like in this year's offerings, but the Coen brothers' Inside Llewyn Davis tops my list of competition contenders so far, writes Xan Brooks